Electrostatic screen printing of articles made of highly insulating materials



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ELECTROSTATIC SCREEN PRINTING OF ARTIULES MADE OF HIGHLY INSULATING MATERIALS Filed July 27, 1964 INVENTOM KENNETH u). Rama,

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United States Patent ELECTROSTATIC SCREEN PRINTING 0F ARTI- CLES MADE OF HIGHLY INSULATING MA- TERIALS Kenneth W. Rarey, South Holland, John B. Kennedy, Jr.,

Chicago, and James G. Buck, Western Springs, Ill., assignors to Continental Can Company, Inc., New York, N.Y., a corporation of New York Filed July 27, 1964, Ser. No. 385,431 Claims. (Cl. 101--114) This invention relates to improved methods of electrostatic screen printing and more particularly to electrostatic screen printing of articles made of highly insulating materials.

In a previously known method of electrostatic printing, a direct current high voltage power supply is connected between a stencil-supporting electrically conductive screen and a co-extensive conductive sheet, which are arranged essentially parallel to each other, to establish and maintain an electric field between the screen and the sheet. A finely divided, low melting temperature, pigmented or dyed, electrically insulating, solid material known as toner is then applied to the exposed side of the screen, generally from a roller of a type similar to that used for painting. The toner becomes electrically charged while passing through unblocked apertures in the screen. The toner experiences a force, due to the electric field between the screen and sheet, which accelerates the toner until it is intercepted by the surface being printed. When a sufficient amount of toner has been deposited in this manner, the object may be removed from its location near the screen. The electrical charge on the toner tends to bind it to the surface on which it is deposited. Heating or exposure to vapor of a solvent for the toner may then permanently bond the toner to the surface.

The electrical functions that are accomplished by the power supply, screen and sheet, are of particular interest. The screen and sheet, when arranged for printing, essentially constitute a parallel plate capacitor. When the power supply is connected between the two, the capacitor becomes charged. As the toner is pushed through the unblocked screen apertures by the roller, some of the charge on the screen flows onto the toner particles, and, as they are carried to the surface being printed, additional charge flows onto the screen. Thus, the power supply provides the electrical charge for the toner. Then, the electric field accelerates the charged toner across the separation between the screen and the object to be printed upon. As the charged toner accumulates on the surface being printed, an electrical repulsion is established which tends to prevent additional deposition. If the object is suificiently conductive, it may be connected to the conductive sheet and accumulation of an electrical charge will be avoided. In this event, electrical conduction between the power supply and object permits formation of a surface charging on the object, in the vicinity of, and of opposite polarity to the charge of the toner. This charging tends to neutralize the field of the toner. Thus, the power supply is providing charge to the object being printed. If the object is not sufficiently conductive to permit this type of operation, the best one can do is to establish such an intense field between the screen and conductive sheet that it is always large compared to that associated with a printed image. Although this can result in the printing of a good image, the image may not be stable when removed from the electric field. Frequently such images will be so unstable that considerable amounts of toner will be ejected from the surface of the printed objects before the images can be fused.

This previously known form of electrostatic screen printing is particularly well suited to the printing of sheet materials in either a planar form or arranged into a 3,296,963 Patented Jan. 10, 1967 simple shape, such as sections of a cone or cylinder. However, it is necessary, if the objects are not good electrical conductors, that a sheet electrode, or some equivalent, be used. The charge available at the screen must be sufiicient to adequately charge the toner. The electric field must be of an appropriate magnitude and direction to transport the toner to the object and form a faithful reproduction of the stencil. Finally, either charge must be added to the object being printed or the electric field must be sufficiently intense to overcome the electric field due to accumulating toner on the image, such that the paths of subsequently arriving particles will not be appreciably influenced.

It is apparent that all electrically insulating objects that require printing cannot accommodate an equivalent to the sheet electrode. A narrow necked plastic bottle having a shape which is not restricted to cones or cylinders is an example of such an object. In fact, most plastic bottles fall into this category, especially if one includes consideration of all portions of the bottle. A narrow neck and complex shape prohibits locating an electrode in close proximity to the inner surface. The complex shape may mean that a simple rod electrode located in the center of the bottle cannot provide an electric field of appropriate direction and magnitude at all points to avoid distorting the pattern. In addition, it may not be possible to get a sufficient quantity of charge at the screen, and field intensity near the screen, to provide adequate toner charging. An electrode located outside of the bottle, on the side opposite that of the screen, may exhibit similar deficiencies, and additionally, restrict the amount of area that can be printed at one time, thus reducing printing speed.

In the present invention, electrostatic screen printing is accomplished in a manner which is free of the deficiencies inherent in the previously known methods.

In accordance with the present invention, a bulky object made of an insulating material such as, but not limited to, a sheet of plastic or a plastic bottle, with thin walls, is exposed simultaneously to two corona discharges, one on each side of the object or, in the case of a hollow article, one inside and the other outside. The two corona discharges deposit, on the opposite surfaces, ions of the opposite electrical polarity. These two surface charges, being located closely together and being of opposite polarity, tend to neutralize the electric fields of each other in the space near the objects. Now, if an electrically conductive stencil-bearing screen is brought sufiiciently near the external surface of the object, an enhancement of the electric field in the space between the two occurs. This is because the field, due to the static charging of the article, induces charging on the conductive screen which tends to produce field intensification. if the screen is electrically grounded, this effect may be further enhanced.

It is an object of the present invention to provide a new and novel method of electrostatic screen printing upon an article of highly insulating material, the method comprising the steps of charging one side of the article to be printed with positive ions and and the other side of the article with negative ions to provide the article with a distributed electrostatic charge, locating the charged article in spaced relation to an electrically conductive screen to establish an electric field between the charged article and the conductive screen, and forcing toner particles through apertures in the conductive screen to charge the toner particles and transport them to the charged article.

A further object of this invention is to provide a method, of the type described above, including the further steps of placing an electrode adjacent to the side of the article remote from the conductive screen, connecting the electrode to the conductive screen, and electrically grounding the conductive screen to enhance the field intensity of the electric field between the conductive screen and the charged article.

Another object of this invention is to provide a new and novel method of coating an exterior surface of a hollow article of non-conductive material which comprises the steps of creating in spaced relation from and substantially within the boundaries ofthe interior surface of the hollow article, a localized zone of corona discharge of predetermined polarity to coat the interior surfaces with ions of a polarity determined by said predetermined polarity, creating in spaced relation from the exterior surface of the article a second zone of corona discharge of opposite polarity from that ofthe corona discharge within the boundaries of the interior surface of the hollow article, disposing the charged hollow article in spaced relation relative to an electrically conductive screen stencil to establish an electric field between the hollow article and the screen stencil, and forcing toner particles through apertures in the screen stencil. 7

Another object of this invention is to provide apparatus for printing on an article of highly insulating material having opposite surfaces thereof charged with ions of opposite polarity, the apparatus compr'isingan electrode to be disposed in spaced relation adjacent to one surface of the article, an electrically conductive stencil screen to be disposed in spaced relation adjacent to the other surface of the article, means connectingthe electrode to the stencil screen, and means for forcing toner particles through apertures in thestencil screen. H I

A further objectof this invention is to provide apparatus, of the type set forth above, wherein the means for forcing toner particles through apertures in the stencil screen comprises a magnetic brush for brushing toner particles into and through the apertures in the stencil screen.

A still further object of this invention is to provide apparatus, of the type set forth above, wherein the magnetic brush comprises a bar magnet and a plurality of ferromagnetic particles adhering thereto, the ferromag-- netic particles having smaller size toner particles triboelectrically adhering thereto.

With the above and other objects in view that will hereinafter appear, the nature of the invention will be more clearly understood by reference to the following detailed description, the appended claimed subject matter and the several views illustrated in the accompanying drawing. I

In the drawing: 1

FIGURE 1 is a diagrammatical view of one form of the present invention wherein a sheet of insulating material is illustrated as being exposed to two simultaneous corona discharges of opposite polarity.

FIGURE 2 diagrammatically illustrates a printing operation in accordance with the present invention.

FIGURE 3 is a diagrammatical view, similar to FIG- URE 1, andillustrates a hollow article of insulating material being exposed to two simultaneous corona .discharges, one discharge occurring within the confines of the hollow article and the other discharge occurring exteriorly thereof.

FIGURE 4 is a fragmentary perspective view of a plas- ,tic necked-bottle, with portions broken away for clarity,

and illustrates the plastic bottle ashaving a coating of negative ions on the interior surface thereof, and a coating of positive ions on the exterior surface thereof.

FIGURE 5 is a diagrammatical view of one arrangement for printing upon a hollow article in accordance with the present invention.

FIGURE 6 illustrates a modified arrangement for printing upon articles in accordance with the present invention. a

One form of a practice of the present invention is illustrated in FIGURES 1 and 2. A flat sheet or article 10, which article is formed of a highly insulating material such as acetate film or plastics, is shown as being disposed between a pair of spaced corona electrodes 11 and 12. The corona electrode 11 is electrically grounded by a conductor 13, and the corona electrode 12 is connected to a Van der Graaf generator, generally indicated by the numeral 14. In one operative embodiment of the present invention, the corona electrodes 11 and 12 were common sewing pins which were located about 2 or 3 inches apart with the points directed toward each other. The generator 14 was operated at around 100 kilovolts or more, and the flat article was passed back and forth between the electrodes 11 and 12 so that ions formed in corona discharges near the points were deposited on the article 10 in a fairly uniform manner. Positive ions came from the discharge caused by the grounded electrode 11 and negative ions were associated with the electrode 12 connected to the generator 14. The minus signs 15 indicate that negative ions are deposited on one side of the article It), and the plus signs 16 indicate a coating of positive ions on the opposite side of the article 10.

After the article 10 has been charged by the apparatus shown in FIGURE 1, the article 10 is located in spaced relation to an electrically conductive stencil screen 20 such that an electric field is established between the charged article 10 and the stencil screen 20. The stencil screen 20 is illustrated as being a conventional type screen, which is commonly used in silk-screen printing, and includes a suitable discontinuous coating 21 which has openings 22 therethrough to define a predetermined pattern. The stencil screen 20 is, preferably, electrically grounded as at 25. By disposing the electrically conductive stencil screen 20 sufficiently near the charged article 10, an enhancement of the electric field occurs in the space between the stencil screen 20 and the charged article 10. This is because the field due to the static charging of the article 10 induces charges 23 on the conductive stencil screen 20 which thus produces field intensification. It is preferable that the stencil screen 20 be electrically grounded because such arrangement will further intensify the electric field.

The ink which is used in the presently described printing process is, preferably, a toner of non-conductive particles which are of commercially available types manufactured by Xerox Corporation. The toner consists essentially of pigmented or dyed particles of a mixture of nbutyl methacrylate (41%) and polystyrene (59%), with an average particle size of about 17 microns.

One method of particular interest for charging the toner involves mixing the toner with another granular material known as a carrier. The carrier is comprised of conductive particles which, preferably, consist of iron filings or other magnetic material, but which may be of various materials such as steel, aluminum or copper. Carrier particles have a size which ranges from around to 1000 microns for these various materials. The mixture of toner and carrier particles is generally known as developer. When the toner and carrier are mixed, triboelectric charging occurs and the two types of particles attract each other such that the toner, being much the smaller of the two, forms a particulate coating on the carrier. Care must be taken to insure that an excessive amount of toner is not used in such a mixture. Once coatings are formed on all of the carrier particles, any additional toner simply tends to agglomerate and may exhibit net charging of either polarity. Even if a proper amount of toner is used in such a mixture, care must be taken in mixing to insure a homogeneous distribution. When correct quantities of toner and carrier are properly mixed, the toner essentially all exhibits net charging of a single polarity, and of sufiicient magnitude to be useful in an electrostatic printing process. A desirable feature of such a mixture is that the toner is well contained and does not present a dusting problem generally otherwise associated wit-h such powders. If the carrier is made of a ferromagnetic material, and such material is preferred, magnetic fields can be used advantageously in handling the developer. As is shown in FIGURE 2, a magnetic brush, generally indicated by the.

numeral 30, comprises a simple bar magnet 31, one end of which has been previously dipped into a carrier-toner mixture. Upon withdrawal therefrom, long filaments 32 of carrier particles coated with toner extend from the magnet 31 somewhat as the bristles on a paint brush. The magnetic brush 30 is used to push toner through the screen apertures 22 which are not blocked by the stencil coating 21. The mesh of the stencil screen 20 is such as to permit passage of small toner particles but prevent passage of the larger carrier particles. The magnetic brush 30 is reciprocated, as indicated by the arrows in FIGURE 2 such that toner particles 34, having a negative charge thereon, are passed through the stencil screen 20 and are moved toward the charged article by the electric field between the stencil screen and the charged object 10. The brushing operation is continued until a sufficient amount of toner has been accumulated on the article 10.

The article 10 is now removed from its location near the screen, and is either heated or exposed to vapor of an appropriate solvent until the printed pattern is adhered to the article. The charging of the article 10, of course, maintains the pattern on the object until it is permanently adhered.

The invention is not limited to printing upon fiat sheets, such as the article 10, and, in fact, is particularly adapted for printing upon a bulky article made of an insulating material, such as the thin-walled plastic bottle 40 which is shown in FIGURE 3. The article 40 is charged in the same manner as the article 10. Corona electrodes 11 and 12 are connectedto a Van der Graaf generator 14 operating at around 100 kilovolts or more. It is preferred, but not restricted thereto, that the interior surface 41, of the article 40, be coated with ions of negative polarity, from the corona electrode 11, and the exterior surface 42 be coated with positive ions from the corona electrode 12. Of course, the electrical resistivity of the article 40 must be sufficiently high to maintain a high level of charging throughout the subsequent printing operation.

As is shown in FIGURE 5, an electrically conductive stencil screen 45 is positioned near the article 40 in an appropriate location for stenciling. The stencil screen 45 is electrically grounded, as at 46, and will charge under the action of the electric field from the precharged hollow article 40 which is to be printed upon. This char-ging of the stencil screen 45 may then serve to charge the toner as it passes through the apertures in the screen stencil 45. The electric field between the screen stencil 45 vand article 40 serves to carry toner from the stencil screen to the article. It is preferred, but not necessary, to place an electrode 47 inside the hollow article 40 and connect the electrode 47 to the electrically grounded stencil screen 45. Such a connection serves to enhance the screenarticle field intensity.

As is illustrated in FIGURE 6, a conventional paint roller Stl, for example, may be used for introducing toner 34 through apertures in the stencil screen 45. The charged toner 34 is then electrically accelerated to the article 40, by the electric field between the article and stencil screen 45, to produce a printed pattern 51 upon the article 40. The magnetic brush 30, as is shown in FIGURE 2, may be substituted for the roller 50 and, as previously pointed out, the use of a magnetic brush is preferred. After the pattern 51 consists of a sufficient amount of toner 34, the article 40 is removed from its location near the screen and is either heated or exposed to vapor of an appropriate solvent until the printed pattern 51 is adhered to the article 40.

While preferred forms and arrangement of parts have been shown in illustrating the invention, and preferred steps of a printing process have been described, it is to be clearly understood that various changes in details and arrangement of parts may be made without departing from the spirit and scope of the invention, as defined in the appended claimed subject matter.

We claim:

1. A method of electrostatic screen printing upon an article of highly insulating material, said method comprising the steps of charging one side of the article to be printed with positive ions and the other side of the article with negative ions to provide said article with a distributed electrostatic charge having a net charge of approximately zero, placing the charged article in proximate spaced relation to an electrically conductive screen while maintaining said distributed electrostatic charge to induce electrical charges on said conductive screen and to establish an electric field between the charged article and the conductive screen, forcing toner particles through apertures in the conductive screen causing said toner particles to be transported by said electric field from said screen to said charged article.

2. A method as defined in claim 1 including the step of prearrangin-g the apertures in the conductive screen to define a predetermined pattern.

3. A method as defined in claim 1 including the further step of fusing the toner particles to the charged article.

4. A method as defined in claim 1 including the further step of electrically grounding the conductive screen.

5. A method as defined in claim 1 including the further steps of placing an electrode adjacent to the side of the article remote from the conductive screen, connecting the electrode to the conductive screen, and electrically grounding the conductive screen to enhance the field intensity of the electric field between the conductive screen and the charged article.

6. A method as defined in claim 5 including the step of pre-arranging the apertures in the conductive screen to define a predetermined pattern.

7. A method of coating an exterior surface of a hollow article of non-conductive material which comprises the steps of creating in spawd relation from and substantially within the boundaries of the interior surface of said hollow article a localized zone of corona discharge of predetermined polarity to coat the interior surfaces with ions of a polarity determined by said predetermined polarity, creating in spaced relation from the exterior surface of the article a second zone of corona discharge of opposite polarity from that of the corona discharge within the boundaries of the interior surface of the hollow article to coat the exterior surfaces with ions, placing the charged hollow article in proximate spaced relation relative to an electrically conductive screen stencil to induce electrical charges on said screen stencil to establish an electric field between the exterior surface of the hollow article and the screen stencil, forcing toner particles through apertures in the screen stencil, and cansing toner particles to be transported by said electric field from said screen to said charged article.

8. A method as defined in claim 7 including the further step of fusing the toner particles to the charged article.

9. A method as defined in claim 7 including the further step of electrically grounding the conductive screen stencil.

10. A method as defined in claim 7 including the further steps of placing an electrode adjacent to the side of the article remote from the conductive screen, connecting the electrode to ground, and electrically grounding the conductive screen to enhance the field intensity of the electric field between the conductive screen and the charged article.

11. Apparatus for printing on an article of highly insulating material, means for charging respective opposite surfaces of said material with charged ions of opposite polarity, an electrode adapted to be disposed in spaced relation adjacent to one surface of the article, an electrically conductive stencil screen adapted to be disposed in spaced relation adjacent to the other surface of the article, means for electrically grounding said electrode and said stencil screen, and means for introducing toner particles into apertures in said stencil screen.

12. Apparatus for printing on an article of highly insulating material, means for charging opposite surfaces of said material with charged ions of opposite polarity, an electrode adapted to be disposed in spaced relation adjacent to one surface of the article, an electrically conductive stencil screen adapted to be disposed in spaced relation adjacent to the other surface of the article, means for electrically grounding said electrode and said stencil screen, and a magnetic brush for brushing toner particles into and through apertures in the stencil screen.

13. Apparatus as defined in claim 12 wherein said magnetic brush comprises a bar magnet and a plurality of ferromagnetic particles adhering thereto, said ferromagnetic particles having smaller size toner particles triboelectrically adhering thereto.

14. Apparatus for printing on an article of highly insulating material having opposite first and second surfaces, means for charging said first surface with ions of a first polarity and said second surface with ions of opposite polarity, an electrically conductive stencil screen means for being disposed in spaced relation to said article after said article is charged, whereby electrical charges will be induced on said conductive stencil screen means to establish an electric field between the charged article and said conductive stencil screen means, said stencil screen means having apertures therein, means for introducing toner particles into said apertures, where-by said toner particles are transported to said article by the electric field between said article and said stencil screen means, an electrode disposed on the side of said article opposite from said stencil screen means and in spaced relation to said article, and means for electrically grounding said electrode and said stencil screen means for enhancing the field intensity of the electric field therebetween.

15. Apparatus as defined in claim 14 wherein said article is a hollow container made of plastic material and having an opening therein, said electrode comprising a rod-like member inserta-ble within said hollow container through said opening,

References Cited by the Examiner UNITED STATES PATENTS 2,874,063 2/1959 Greig 118637 X 2,885,556 5/1959 Gundlach 250-495 2,890,968 6/1959 Giaimo.

2,894,139 7/1959 Magruder et al. 101426 X 2,911,330 11/1959 Clark 10l-426 X 2,922,883 1/1960 Giaimo 25049.5 2,957,077 10/1960 Hay 25049.5 2,965,481 12/1960 Gundlach 250-49.5 2,966,429 12/1960 Darrel et al 11717.5 X 3,017,339 1/1962 Dewey 25049.5 3,058,444 10/1962 Sugarman et a1 118-637 3,064,259 11/1962 Schwertz 118637 3,081,698 3/1963 Childress et al.

3,133,484 5/1964 Wright 118-637 X ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Assistant Examiner. 

1. A METHOD OF ELECTROSTATIC SCREEN PRINTING UPON AN ARTICLE OF HIGHLY INSULATING MATERIAL, SAID METHOD COMPRISING THE STEPS OF CHARGING ONE SIDE OF THE ARTICLE TO BE PRINTED WITH POSITIVE IONS AND THE OTHER SIDE OF THE ARTICLE WITH NEGATIVE IONS TO PROVIDE SAID ARTICLE WITH A DISTRIBUTED ELECTROSTATIC CHARGE HAVING A NET CHARGE OF APPROXIMATELY ZERO, PLACING THE CHARGED ARTICLE IN PROXIMATE SPACED RELATION TO AN ELECTRICALLY CONDUCTIVE SCREEN WHILE MAINTAINING SAID DISTRIBUTED ELECTROSTATIC CHARGE TO INDUCE ELECTRICAL CHARGES ON SAID CONDUCTIVE SCREEN WHILE MAINAN ELECTRIC FIELD BETWEEN THE CHARGED ARTICLE AND THE CONDUCTIVE SCREEN, FORCING TONER PARTICLES THROUGH APERTURES IN THE CONDUCTIVE SCREEN CAUSING SAID TONER PARTICLES TO BE TRANSPORTED BY SAID ELECTRIC FIELD SAID SCREEN TO SAID CHARGED ARTICLE. 